code: def append(self, kv: dict):
      # add ',' if not the first element
      if self._string.getvalue() != "[":
          self._string.write(",")
      json.dump(kv, self._string)
  
file_path: FATE/python/fate/arch/federation/message_queue/_datastream.py
imports: import io
  import json
  import sys
label_desc: Use image node_id 4 for calling the Datastream obj's underlying member method code with example usage: obj.append(kv) without return types